Closed-loop active control of combustion instabilities using subharmonic secondary fuel injection

The results of a study of closed loop active control of combustion instabilities using subharmonic secondary fuel injection are presented. The tests were conducted in a laboratory scale dump combustor at atmospheric pressure with an inlet temperature of 415 degrees C, an inlet bulk velocity of 9.3 m/s, and over a range of fuel lean equivalence ratios from lean blowout to the stoichiometric condition, The observed instability was a longitudinal mode instability with a frequency of similar to 350 Hz. Control was achieved using modulated secondary fuel injection on every fourth cycle of the unstable dame. The injection of secondary fuel was phase locked with the pressure oscillation in the combustor, where the relative phase was adjusted to achieve maximum control. The most effective control was achieved at an equivalence ratio of 0.85, where the combustion-generated noise level was reduced by 22 dB, with a secondary fuel flow rate that was only 2% of the primary fuel flow rate.